help in heat sensing and light sensing

Thread Starter

toto_na16

Joined Jun 30, 2009
11
i have a project that activate an ac fan when the temperature is 25 c or above and there is a light in the room


i tried to design a circuit which consist of heat sensor and light sensor and AND gate and Relay

when i drew heat sensor circuit and simulate it in multisim and used an oscilloscope it give 4.950 v when on and 36.508 mv when it is supposed to be off

and the same in light sensor


when i added them to an AND gate and the two sensors is on it gives exactly 5v


when one is off and one is on it gives also 5v


i attached the three circuits via multisim


http://rapidshare.com/files/250333500/circuit.rar


thank u in advance
 

SgtWookie

Joined Jul 17, 2007
22,230
You should have posted screen prints (.PNG format preferred) instead of putting Multisim format files compressed together in .rar format on a strange server.

Not everyone has Multisim installed.
Not everyone has a .RAR decompression utility installed.
Many aren't willing to download files from another server.

Try using the "Go Advanced" feature and "Attach Files" that are in .png format.
 

DC_Kid

Joined Feb 25, 2008
1,072
yeah, stay away from downloading stuff you dont trust. RAR files from a site named "rapidshare" sounds scary. although i know both well i suggest the avg joe not to open or download anything you dont trust.

sorry, i am just a security (CISSP) guy.....

I've used multisim before, found some odd results with some basic circuits. perhaps try LTspice from LT.
 

SgtWookie

Joined Jul 17, 2007
22,230
1) You do not have a ground symbol on either of your comparator circuits. Place one on each negative rail.

2) Double the values of R4 and R8.

3) R5 and R6 are much too large. 1.5k Ohms is a typical value for a single comparator output pull-up resistor.

4) You could dispense with the AND gate entirely. Connect the outputs of the two comparators together, and use a single 1.5k pull-up resistor. This will result in a wired AND configuration.
 

Thread Starter

toto_na16

Joined Jun 30, 2009
11
i don't understand the note no.1

and the note number 4

i can't remove AND gate because it is a project required component
 

SgtWookie

Joined Jul 17, 2007
22,230
i don't understand the note no.1
1) You do not have a ground symbol on either of your comparator circuits. Place one on each negative rail.

This means to connect a ground symbol to the negative (-) side of each of the batteries, V1 and V2.

and the note number 4

i can't remove AND gate because it is a project required component
OK, so instead use a CMOS 4081 AND gate.

However, your 7408 AND gate may work if you reduce R5 and R6 to 1.5k Ohms.
 

mr boss

Joined Jun 3, 2009
11
I don't know which version of multisim you use, but either way you do need a ground symbol. When I draw a circuit in my multisim, and I simulate it without a ground it gives me a error and stops the simulation immediately
 

Thread Starter

toto_na16

Joined Jun 30, 2009
11
note : i replaced LDR and thermistor with R7 and R1 respectively

i made these changes u said




but thr circuit doesn't do the function

when thermistor become above 10 k the output goes 5V
and when LDR become less than 10 k it goes 5V

that doesn't do the wanted function


note : this an NTC thermistor
 

Thread Starter

toto_na16

Joined Jun 30, 2009
11
I don't know which version of multisim you use, but either way you do need a ground symbol. When I draw a circuit in my multisim, and I simulate it without a ground it gives me a error and stops the simulation immediately
i am using multisim 10.1
 

SgtWookie

Joined Jul 17, 2007
22,230
note : I replaced LDR and thermistor with R7 and R1 respectively

I made these changes that you suggested

No, you did not. Did you even read what I said to do?

but the circuit doesn't do the function

when thermistor become above 10 k the output goes 5V
and when LDR become less than 10 k it goes 5V

that doesn't do the wanted function

note : this an NTC thermistorC
You need to have the static resistors be significantly larger than the variable resistors, otherwise the comparators will never have a trigger point.

R5 and R6 need to be reduced to 1.5k.
 

Thread Starter

toto_na16

Joined Jun 30, 2009
11
ok i am sorry





i doubled R4 and R8 in this circuit as you said

but when the variable resistor R1 become under 100% it is supposed to give an output of 5V but it doesn't do

why ?
 

R!f@@

Joined Apr 2, 2009
9,918
Don't worry guys, I have pretty powerful security and the RS he hosted is legitimate.
I can correct the circuit if all fails cause I have multisim. Just let me know

Rifaa
 

millwood

Joined Dec 31, 1969
0
the circuit you designed will work this way:

1) U1A: if R1 > R4, it outputs 0v; otherwise, it outputs 5v;
2) U2A: if R7 > R8, it will output 5v; otherwise, it outputs 0v;
3) U3A: it outputs 5v if both U1A and U2A output 5v; otherwise, it output 0v.

so in your current design, and with the current resistor values, U1A will always output 5v and U2A will always output 0v; thus U3A will always output 0v.

whether that's what you wanted, I have no idea.
 
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